Season 17, Episode 56: What is the Planck Length and Why Does it Matter?

Hey StarTalkians! Episode 56 of season 17 was a Cosmic Queries edition, with Neil, Chuck, Gary and astrophysicist Charles Liu. About halfway through, they received this mind-bending question:

Cosmic Queries – The Fractal Universe with Charles Liu - StarTalk Special Edition

(From 30:30)

You can tell from the strained response that this isn’t a simple matter, but it all revolves around the so-called Planck length. Beyond this limit, we’re pretty sure our theories break down. But what even is it, and why is it so significant?

Planck’s Units Explained

Planck’s constant is familiar to anyone who’s studied quantum mechanics, but it’s worth starting with Max Planck’s original idea of a new system of units. In an 1899 paper, in a short final section entitled “Natural Units of Measurement,” Planck points out the terrestrial nature of our units of measurement.

Planck was interested in proposing units that would make sense to anyone, even extraterrestrial cultures. To this end, he worked with two constants from the problem he solved in the paper (including what we now call Planck’s constant, h), along with the speed of light in a vacuum (c) and the gravitational constant (now called G).  

By combining these constants in appropriate ways, he derived values for what we now call the Planck length, the Planck mass and Planck time, as well as Planck temperature.

These values diverge from “everyday” human-world values quite a bit. For instance, while the Planck mass is closer to a useful size, the Planck length is unimaginably tiny. As Symmetry magazine explains, if a proton was the size of the observable universe, the Planck length would stretch only from Chicago to Tokyo.

The Physical Significance of Planck Units

Planck was not trying to make some bold prediction about quantum gravity when he proposed his units, but they end up central to these discussions anyway. In the 1950s physicists started to notice the Planck length cropping up in their work on quantum gravity. For example, John Archibald Wheeler investigated quantum fluctuations of the spacetime metric, and found that these fluctuations become significant at the Planck length – even though he didn’t even know about Planck’s units at the time!

As Neil stressed in the podcast, many different results like this indicate that the Planck scale is where our ideas about the universe start to break down. We’d need a theory of quantum gravity to make sense of it.

Season 17, Episode 55: Drake, Seager and Oluseyi – Are We Alone in the Universe?

Hey StarTalkians! Episode 55 of season 17 was a Cosmic Queries edition with astrophysicist Dr. Hakeem Oluseyi. While discussing his new book Why Do We Exist, Dr. Oluseyi mentions his “Oluseyi equation”:

Why Do We Exist with Hakeem Oluseyi - StarTalk Radio

(from 12:30)

Neil immediately compares it to the famous Drake equation. And despite key differences, their goals are ultimately the same. But that goal might not be what you’re thinking.

The Drake Equation Explained

Frank Drake’s equation was proposed in 1961, when the Search for Extraterrestrial Intelligence (SETI) was just getting started. It’s largely composed of a series of probabilities multiplied together.

The equation is for N, the number of civilizations in the Milky Way we may be able to communicate with. It depends on:

• The rate of star formation in the galaxy

• The fraction of stars suitable for life

• The fraction of stars with planets

• The number of planets per system that could support life

• The fraction of these that develop life

• The fraction of that life which is intelligent

• The fraction of intelligent life with the technology and desire to communicate

• How long such a civilization lasts

Dr. Oluseyi points out that this is outdated. We now know, for instance, that basically every star hosts planets. The equation is clearly geared towards the goals of SETI, with a focus on radio communication across space.

Speculation vs. Science

The Drake equation is largely speculative. How can we possibly determine the fraction of planets that develop intelligent life? It was intended more as a conversation-starter than it was a genuine piece of science [2]. The results depend strongly on how optimistic you are about these unknown questions.

Seager’s Variant and the Oluseyi Equation

Dr. Oluseyi mentions Sara Seager’s version. She switches the focus on radio signals for a more modern, grounded focus on life creating a “signature gas” which can be picked up by spectroscopy.

Dr. Oluseyi’s equation from his new book continues this tradition, focusing instead on the presence of multi-cellular life. He describes it as combining the probability of “just right” stars, “just right” planets and whether life actually occurs.

All of these – from Drake to Oluseyi – have a shared goal: to promote discussion of the search for alien life. It’s more about sparking thought than it is getting a solid number.  

Season 17, Episode 54: Galileo Proved That King Kong Would Struggle to Stand

Hey StarTalkians! Episode 54 of season 17 was a “Thing You Thought You Knew” edition, where Neil gave Chuck a crash course in sidereal days, impact craters and “the size of life.”

Why Craters Are Round, Size of Life, The Real Length of Day – TYTYK -…

(From 32:20)

The discussion on the scaling of strength and mass for animals hearkens back Galileo – and likely earlier – and leads to one unusual conclusion. Galileo proved that King Kong wouldn’t work about 300 years before the iconic monster appeared on-screen.

From Galileo to On Being the Right Size

Galileo probably came up with his idea because of a mistake he made when giving a lecture about Dante’s Inferno and the huge structure it implies. His mistake is a pretty intuitive one: if you take a structure and scale everything up equally, you’d naively expect it to still hold up just fine.

But Galileo realized there’s a problem. Imagine a chandelier hanging from a small rope that can just about hold it up. If you double every dimension of the chandelier, the volume – and mass – increases by 2 × 2 × 2 = 8 times. But the rope’s strength only comes from its cross-sectional area, i.e. the number of strands it contains. If you double every dimension of the rope, the length is useless and the strength only increases by 2 × 2 = 4 times.

The chandelier got heavier quicker than the rope got stronger, and it would come crashing down.

A famous essay by polymath J. B. S. Haldane addresses “being the right size,” and wraps Galileo’s conclusions up in a quirky-yet-informative package. He discusses some fictional giants that were ten times a human’s size in each dimension. This means 1,000 times the mass. But their legs – like the rope above – only get 100 times stronger. This means each leg would need to be working 10 times harder in a proportionally-scaled giant.  

Why King Kong is (Probably) Impossible

King Kong’s size varies in different movies, but 50 ft is an oft-used figure. This makes Kong 8.33 times bigger than a normal gorilla. This means his legs would be about 69 times stronger, but he’d weigh about 578 times more. Based on a 150 kg gorilla, Kong would weigh almost 87,000 kg, in the range of the largest sauropod dinosaurs, which had proportionally much wider legs. Even if he could stand, he certainly wouldn’t be climbing any skyscrapers.

Season 17, Episode 51: 3 Ways We’re Hunting Dark Energy, Explained

Hey StarTalkians! Episode 51 of season 17 welcomed Dr. Jason Rhodes, who sat down with Neil and Paul to discuss the new Nancy Grace Roman Space Telescope and the hunt for dark energy. This took up most of the podcast, but particularly the first half:

The Hunt for Dark Energy with Jason Rhodes - StarTalk Radio

Dr. Rhodes mentions three ways the telescope will be looking for dark energy, but as is often the case, it was easy to get lost in the meandering discussion. So here’s a briefer, clearer list.

Method 1: Type 1a Supernovae

This was discussed early on in the history of this board, but the short version is: these supernovae are “standard candles,” which are used to fix cosmic distances. If you fix the distances of many of these across different redshifts, you can tell how fast space appears to be expanding.

Method 2: Baryon Acoustic Oscillations

Baryon acoustic oscillations are a little like frozen sound waves, spread out over the universe. Back when the universe was a hot, electron-baryon plasma, photons were basically trapped. They continually interacted with the plasma, until the universe cooled enough for the particles to form into the first atoms. At this point, the photons could finally escape, leading to the well-known cosmic microwave background radiation (CMB).

But at the same time, the ripples of what happened in the plasma phase spread out too. Denser regions of the early plasma universe were pulled together by gravity, but pushed back apart by the interacting photons. This led to oscillations – like sound waves – which travelled through the plasma until the universe cooled.

These waves were released along with the CMB, and form another key relic of the early universe. A distinctive signal of this has been detected.

Like type 1a supernovae, these can be used to establish a distance scale, and any distance scale can be used to probe dark energy.

Method 3: Weak Lensing

Gravitational lensing is usually discussed in the dramatic cases, like a galaxy cluster magnifying and warping an ancient galaxy behind it. But it also operates on much smaller scales, and this type is useful in the hunts for dark matter and dark energy. This gets especially technical, but the short version is: weak lensing is useful in combination with the other two methods, and by helping to map the distribution of dark matter.

Season 17, Episode 52: Rogue Planets Explained

Hey StarTalkians! Episode 52 of season 17 was a Cosmic Queries edition, featuring Neil, Chuck and guest Professor David Kipping discussing all things planetary. About half way through the podcast, Chuck set the ball rolling on an interesting discussion about wandering “rogue planets”:

Rogue Planets & Exomoons with David Kipping - StarTalk Radio

(from 31:30)

There are a lot of threads that could be picked up there – the history of our own solar system, for one – but I left mainly wanting to know more about the search for rogue planets. So here’s a quick primer.

How Planets Go Rogue

Although there are several ways planets can become rogue, the simplest one is outlined in a paper from the 90s focusing on two Jupiter-sized planets.

If the planets evolved at different rates, or if they continue gaining mass, it can lead to instability in their orbits and interactions between them. The authors found that when they didn’t collide (about 50% of cases), one planet tended to be ejected and become a rogue, while the other settled into a tighter, more squashed-out orbit. When they also included smaller planets, these often got ejected from the system too.

How We Find Rogue Planets

Microlensing is a common approach to finding these rogues, as discussed on the podcast. Gravitational lensing happens on small scales too, and sensitive-enough detectors – as the Nancy Grace Roman Space Telescope will have – can pick up lensing caused by smaller objects.

Even though rogue planets don’t give off light, their gravity does alter the light profile from stars. Most of the work on rogue planets has involved careful observation of a star for the characteristic bump in brightness from microlensing.

Low Mass Rogues

The challenge is spotting this small effect, because for a rogue planet, it won’t repeat. In fact, the length of time this goes on for is a key metric for inferring the size of the object. A 2020 paper described the (not cool-y named) OGLE-2016-BLG-1928 event, which only lasted for about 41 and a half minutes. The main result (see images) shows just how hard this is to catch. This object is assumed to be near Mars’ mass.

How Many Rogue Planets Are There?

One recent estimate suggests that there are about 20 rogue planets per star in our galaxy, and that the Roman space telescope will find something around 1,000 of them.

Season 17, Episode 48: 3 Ways Space Weather (Possibly) Impacts Your Everyday Life

Hey StarTalkians! Season 17, episode 48 was a sit-down discussion with Neil, Chuck and guest Dr. Lika Guhathakurta, a heliophysicist at NASA. The latter half addressed space weather, coronal mass ejections and the many ways in which the sun can – and does – impact our lives.

Super Solar Storms with Lika Guhathakurta - StarTalk Radio

One of Dr. Guhathakurta’s papers delves into what she calls “everyday” space weather, and overlaps with a lot of the podcast discussion.

Carrington-Like Events

Around 50 minutes into the show, they discuss the so-called “Carrington Event” of 1859, and Dr. Guhathakurta also covers this issue in the paper mentioned above. But as her paper points out, there have been others.

A 1989 space weather event knocked out transformers in New Jersey, the United Kingdom and Quebec, with Quebec also losing power for 9 hours. In 2003, geomagnetic storms caused blackouts in southern Sweden and possibly damaged transformers in South Africa.

3 Everyday Effects of Space Weather

Those events are more dramatic – thankfully, we don’t have that type of thing every day. However, that doesn’t mean space weather doesn’t have everyday impacts.

1: Power Problems

It doesn’t have to be a Carrington-like event for space weather to impact your electricity supply. A paper referenced by Dr. Guhathakurta looked at data on insurance claims for industrial electrical equipment, cross-referenced with space weather conditions. While they struggle to pin down a precise dollar amount for the cost – it’s probably in the billions – they calculate that 500 claims are submitted in the US annually because of space weather.

2: Cosmic Rays During Flights

When you travel by plane, you’re exposed to more cosmic rays – high-energy particles originating in distant events like supernovae. At solar maximum, the sun’s magnetic field actually protects us, but at times of lower solar activity, we’re exposed to more. A study looking at this calculated that flying at high latitudes for 12 and a half hours exposes you to as much radiation as a chest X-ray, or about half that for mid-latitudes.

3: The “Blurry Vision” of Your Satellite Navigation

Dr. Guhathakurta estimates [2] that about a third of the inaccuracy of satellite navigation devices come down to space weather. Irregularities in the ionosphere can refract and scatter their signals, leading to what she calls “blurry vision.” It isn’t a huge effect, but it is definitely something you’ll notice often!  

Season 17, Episode 49: How Scientists Are Trying to Eliminate Mosquitoes

Hey StarTalkians! Episode 49 of season 17 switched astronomy for entomology, as Neil and Negin sat down with entomologist Dr. Jessica Ware to discuss all things bugs. I was pretty surprised to hear any biologist support the eradication of any species, but Dr. Ware did so for two mosquito species:

It’s Bugs’ World and We’re Just Living In It with Jessica Ware - StarTalk Radio

(from 37 minutes)

She even mentions a couple of methods scientists have tried. But would they work? Why is there apparently little concern about the ecological consequences?

The Mosquitoes That Kill

It’s not all mosquitoes! The Anopheles gambiae complex – actually seven very similar species – and Aedes aegypti are the main vectors for dengue fever, malaria, yellow fever and chikungunya, and therefore a common target of these efforts.

Irradiated Mosquitoes

As Dr. Ware mentioned, one of the most common methods tried so far releases sterile males into the population to mate and reduce the number of offspring in the next generation.

One test of this in Brazil sterilized the males with radiation, and then released them into the wild population using drones. This was effective, and as a different, self-explanatory paper emphasized, “Yes, Irradiated Sterile Male Mosquitoes Can Be Sexually Competitive!”.

The Bacteria Method: Wolbachia to the Rescue?

Another approach Dr. Ware mentioned involves bacteria called Wolbachia pipientis and targets the A. aegypti mosquito. Specifically, if they are infected with some strains, it prevents viruses such as dengue from growing in their bodies.

A randomized trial released wolbachia-infected mosquitoes into 12 randomly-chosen urban subdivisions in Indonesia. The remaining areas continued practicing ordinary mosquito control measures. They checked people with fevers presenting to government-run health clinics to see if it reduced dengue infection.

They found that people in areas with infected mosquitoes were significantly less likely to be diagnosed with dengue (2.3% vs. 9.4%), and were also less likely to be hospitalized.

Would it Effect the Ecosystem?

As a paper looking at this concluded, for the A. gambiae mosquitoes, most predators that eat these mosquitoes would likely easily substitute other food sources if they were removed. While the Wolbachia method would probably be ecologically benign – since the population is maintained – even population reduction in targeted species seem like it would have minimal impact.

Season 17, Episode 45: What’s Your Real Zodiac Sign? (According to Astronomy)

Hey StarTalkians! Episode 45 of season 17 was a “Things You Thought You Knew” edition, where Neil explains common astronomical misperceptions to a usually bewildered Chuck. The title-fact comes from this discussion at the end of the episode:

Things You Thought You Knew – Your Zodiac is Wrong - StarTalk Radio

(from 45:37)

Neil’s explanation was great, but as an astronomer, he – understandably – didn’t know exactly how this affected Chuck’s zodiac sign. So here’s your new sign, along with a brief look at the physics.

Mandatory Astrology Disclaimer

Suffice it to say that the miniscule gravitational interaction between a distant constellation and you at the moment of your birth has no bearing whatsoever on your personality or what happens in your life.

Astronomical Precession and the Shifting of Zodiac Signs

The discussion of precession in the podcast mainly focused on the changing “north star” over time, but it’s the same idea. If you imagine a very (very!) large physical pole sticking out of the pole of the Earth, the tip would trace out a circle over a period of around 25,800 years.

Since the Earth has an equatorial bulge, there’s slightly more mass at the equator, and the Sun’s (and Moon’s) gravitational influence isn’t evenly spread across the Earth. This creates an effect a little like the wobbling of a spinning top as it slows down.

This slowly changes the point in the orbit where we get solstices and equinoxes. The solstice happens when the axis of the Earth points towards the sun. But precession means that this happens slightly earlier in our orbit each year, since the axis itself is also moving. The season-based “tropical year” is about 20 minutes shorter than the “sidereal year,” measured relative to fixed star locations.

Astrologers over two millennia ago didn’t really take account for this – they effectively assumed a sidereal year, where zodiac signs would remain fixed over time. But they actually cycle around with precession.

Plus, even the Babylonians knew there were actually 13 signs, including Ophiuchus as mentioned on the show. But they stuck to 12 signs, and split the year equally between them.

Your Accurate Star Sign

So if you like your pseudoscience to be as accurate as possible, check the pictures. For Chuck, his July 10th birthday would mean that he’s a Gemini, not a Cancer! Also note that there are only 6 days you could really be a Scorpio.  

Season 17, Episode 46: How Likely Are You to Get Hit By a Meteorite?

Hey StarTalkians! Episode 46 of season 17 had Neil and Chuck tackling some Cosmic Queries, including this challenging question about meteorite impacts:

Cosmic Queries – Planet 9 - StarTalk Radio

(From 39:30)

The question mentioned the unique story of Ann Hodges, one of the only recorded cases of someone getting hit. Neil avoided estimating the odds without doing some calculations, but what type of numbers have people come up with? How likely is it?

The Story of Ann Hodges

Ann Hodges was taking an afternoon nap on her couch on November 30th, 1954 when a fragment of a meteorite crashed through her roof, smashed into her radio and struck her in the thigh. The meteorite was a stony, chondrite type and weighed 3.86 kg (8.5 lb), and left a serious bruise (see images). But other than some (very understandable) anxiety, she was OK.

But as the question pointed out, it seems kind of weird that this type of thing doesn’t happen more.

What is the “Meteorite Flux” of the Earth?

The question really revolves around what’s called the meteorite “flux” of the Earth – that is, the rate at which meteorites hit the Earth.

A 1996 paper offers a couple of good estimates for meteorites weighing over 10 g. The paper’s estimate is around 95 per million square kilometres per year, but an earlier estimate puts it at 83. For bigger meteorites, like the one that hit Ann, it’s about 8.7 per million square km or even less.

How Likely Are You to Get Hit?

This is spread evenly over the surface of the Earth, but – as Neil points out – the population is not. In fact, 95% of the population occupies just 10% of the land area.

So including anything above 10 g, about 1,236 meteorites fall on this more populated area per year. For meteorites above 1 kg, it’s more like 130 per year.

Even counting smaller meteorites, this makes it about a 1 in 12,000 chance that there will be an impact in any square km in one year. And about 530 people will occupy that space. Even if every single strike hit someone, that puts the odds at less than 1 in 6 million for each person per year. And even that is an overestimate, since most would still miss.

Other estimates are a little higher, with astronomer Dr. Clark Chapman putting it at about 1 in 1.6 million over a lifetime.  

Season 17, Episode 43: The “Little Red Dot” Mystery (and Their Little Blue Companions)

Hey StarTalkians! Season 17, Episode 43 was another Cosmic Queries edition, with Neil and Chuck sitting down alongside Dr. Michelle Thaller to discuss all things astrophysics. Towards the end of the episode, they got talking about so-called “little red dots”:

Running Out of Matter with Michelle Thaller - StarTalk Radio

(From 52:30)

Ever since the data started coming in from the James Webb Space Telescope (JWST), these have been a major mystery in astrophysics. Are they active galactic nuclei, “black hole stars,” or something else entirely?

Discovering Little Red Dots

As Neil pointed out in the discussion, the JWST was purposefully built to look for star formation in the early universe. “Little red dots” were spotted in the first data release from the telescope: compact, early-universe objects with fast-moving hydrogen gas and a unique “V”-shaped emission spectrum.

Initial papers assumed that they were densely packed early galaxies. This explained the V-shaped spectrum, but it seemed unlikely that so many stars could have formed so quickly.

Others suspected they were active galactic nuclei – supermassive black holes. This idea explains the fast-moving hydrogen, and the surrounding accretion disc would absorb ultra-violet (UV) light and emit it at longer wavelengths, explaining the red coloration. But they don’t seem to give off enough X-rays for this, and it also implies extremely large masses.

Black Hole Stars

One compelling explanation is that they’re black hole stars. These are black holes “cocooned” in partially-ionized gas. This gas would absorb the X-rays and re-emit them at lower energies, fixing a key problem with the black hole explanation. The dense gas explains the central dip in the V-shaped spectrum, and a rapid accretion rate reduces the estimated mass.

A recent paper presents an analysis of a specific little red dot (GLIMPSE-17775) that matches with the black hole star scenario, including hallmark signs of electron scattering in a dense, ionized gas.  

Little Blue Companions

Another 2026 paper adds to this picture, suggesting that these black hole stars are often accompanied by UV-emitting companions. The paper looked at 83 little red dots – some aided by gravitational lensing – finding that 43% of them had companions, rising to 80% for the brightest dots. These companions help to explain the higher-energy part of the little red dots’ V-shaped spectrum.

They’re also huge – around a billion times the mass of the sun – and are likely star clusters or early galaxies.